1. Field of the Invention
The present invention relates to an improved process for producing bonded activated carbon structures wherein the activated carbon particles are bonded utilizing polyolefin resin microfine powders. The process utilizes microwave radiation to heat a mixture of the activated carbon and polyolefin resin binder in a microwave transparent container. The heated mixture is compacted and cooled to maximize point bond formation within the structure.
2. Description of the Prior Art
Bonded adsorbent structures are known and have been used for numerous industrial applications. Bonded adsorbent structures wherein the adsorbent particles, including activated carbon, are coated with a polymeric binder are disclosed in U.S. Pat. No. 3,091,550. Bonded carbon articles, including shaped pellets and larger articles produced therefrom, are also disclosed in U.S. Pat. No. 3,474,600.
U.S. Pat. No. 3,721,072 discloses a filter device having granulated activated carbon bonded into a unitary mass with a bonding agent which occupies no more than one-half the available space between the carbon granules.
Numerous other references, including U.S. Pat. Nos. 4,061,807; 4,664,683; 4,665,050; 4,753,728; 5,019,311; 5,033,465; 5,078,132; 5,147,722; and 5,331,037, relate to various aspects of bonding individual adsorbent particles to one another using adherent binders.
The technique of bonding contiguous adsorbent particles through a binder particle xe2x80x9cbridgexe2x80x9d to create a unitary self-supporting structure is referred to as point-bonding. While numerous processes involving the application of heat, pressure and, in some cases, shear have been developed for the production of point-bonded articles, there is a continuing need by the industry for improved manufacturing procedures. For example, in the manufacture of absorbent canisters, such as in water filters or automotive evaporative emission canisters, whereby the activated carbon particles are commonly employed to adsorb organic molecules, hydrocarbons or other vapors. Since most emission canisters are designed with partition plates and similar structural components to lengthen the path of the vapors as they pass through that activated carbon, it is impractical utilizing heretofore known technology to manufacture canisters wherein the activated carbon adsorbent bed is a unitary bonded structure. As a result, most canisters utilize a loose bed of the adsorbent particles which are prone to channeling and the production of fines, both of which reduce vapor adsorption efficiency.
The effects of microwaves on the surface properties of active carbon in terms of acidity or basic functionally changes have been studied. Also, microwave energy has been used for drying and regeneration of activated carbons and other powder materials. In still other instances polymer composites or mixtures have been formed or heated with microwave energy. For example, in British Patent Specification 1340503, compacted sintered mixtures of polytetroflouroethylene powder and carbon powder, e.g., powdered graphite or coke flour, have been subjected to microwave radiation to form cohesive products. The discs were placed in a 240 MHz microwave field and heated for 10 minutes. Moore, et al., in Ceram. Eng. Sci. Proc. (1993), 14(9-10), 848-855 report the use of microwave energy to remove polymethyl methacrylate binder from alumina compacts to produce very clean ceramic substrates. The regeneration of activated carbon to remove volatile nonpolar organic substances and foulants adsorbed in gold recovery processes have also been respectively disclosed by Ning, et al., Chinese Patent No. 1277891 and Bradshaw, et al., in J.S. Afr. Inst. Min. Metall. (1998), 98(4), 201-210.
With the improved process of the invention, it is now possible to rapidly and continuously produce bonded activated carbon adsorbent structures. Furthermore, it is possible to produce these structures in a variety of configurations and, if desired, in a container or housing which becomes part of the final structure. These and other advantages will be apparent from the description of the invention which follows.
The invention relates to a process for producing bonded activated carbon structures. The improved process comprises (a) preparing a substantially homogeneous particulate mixture by combining 5 to 30 weight percent, based on the total weight of the mixture, of a polyolefin resin powder having an average particle size between 5 and 125 microns and 70 to 95 weight percent, based on the total weight of the mixture of a particulate activated carbon; (b) introducing the particulate mixture prepared in (a) into a microwave transparent container; (c) exposing the container containing the particulate mixture to microwave radiation to increase the temperature of the activated carbon particles 3 to 30xc2x0 C. above the crystalline melting point of the polyolefin resin; (d) compacting the particulate mixture to increase contact between the particles; and (e) lowering the temperature of the particulate mixture below the melting point of the polyolefin while maintaining point bond formation conditions.
In one aspect of the invention the bonded activated carbon structure is utilized with the container in which it is formed. The bonded adsorbent bed and container are the finished article or a component of the finished article. In another embodiment the bonded activated carbon structure is separated and removed from the container in which it is formed. In this latter case the container simply functions as a mold.
Especially useful bonded structures are produced utilizing ethylene homopolymer, propylene homopolymer, and ethylene or propylene copolymer binder powders, particularly microfine polyethylene and ethylene-propylene copolymers wherein the particles are spheroidal or substantially spheroidal in shape. It is particularly advantageous when the particulate mixture is formed with binders of the above types and granular activated carbon having an average particle size of 5 to 300 mesh. Preferred particulate mixtures contain 75 to 92.5 weight percent of the activated carbon and 7.5 to 25 weight percent of the polyolefin resin binder.
The activated carbon particles are bonded in fixed relationship to each other by the polyolefin resin binder. This is accomplished without significantly reducing the hydrocarbon adsorption capacity of the activated carbon particles and without substantially reducing the vapor permeability of the bonded mass. As a result of each particle being fixed in position relative to adjacent particles, movement of the particles can result in the formation of fines and compaction/rearrangement of the adsorbent bed, is eliminated or minimized. Other advantages of the inventive process include adaptability to rapid, continuous operation and the ability to produce articles of varying shapes and articles wherein the container in which the bonded adsorbent structure is formed is a component part of the finished article. Furthermore, structural components which are part of or positioned within the container, such as partitioning plates or the like, may be molded within the unitary adsorbent bed.
There are also provided articles of manufacture produced by the improved process of the invention. Articles produced by the process of the invention consist of a unitary porous adsorbent structure within a microwave transparent containment means.